Hydrogen peroxide solutions

ABSTRACT

It would be desirable to replace nitric acid based solutions for surface treating steels and like materials with a sulphuric acid based solution containing hydrogen peroxide, but such replacement solutions lose hydrogen peroxide rapidly through mainly iron-induced decomposition. 
     A surface treatment solution that is based on sulphuric acid and hydrogen peroxide, but has improved stability, contains an effective amount in combination of hydrofluoric acid, are hydroxybenzoic acid and an N-alkoxyphenyl-acetamide. Preferably, the hydroxybenzoic acid is para-hydroxybenzoic acid and the N-alkoxyphenyl-acetamide is N-(4-ethoxyphenyl)-acetamide. It is preferable to employ a saturated solution of each of the two latter components, and this can achieved practically and simply by adding the solid components in the shape of a block or blocks which maintain the saturated solution over an extended period of time.

The present invention relates to hydrogen peroxide solutions and moreparticularly to a process for its stabilisation. In a further aspect thepresent invention also relates to a stabiliser system for aqueoushydrogen peroxide solutions that are intended for use in metal surfacetreatments.

BACKGROUND OF THE INVENTION

One of the many uses for hydrogen peroxide solutions, and especiallyaqueous acidic hydrogen peroxide solutions, comprises the treatment ofmetal surfaces so as to alter their appearance and to impart chemicallyto the surface a desired sheen or polish. This is often referred tosimply as pickling or polishing. Conventionally, solutions for that usecontain one or more strong acids, which, is normally a mineral acid, aswell as the hydrogen peroxide. In the course of the metal surfacetreatment, there is a tendency for the solution to dissolve metal orimpurities from the metal surface and to strip away particulateparticles that had adhered to the metal surface before the treatmentcommenced. The metals that are pickled or polished usually comprise orcontain at least a proportion of transition metals, such as iron orcopper, which catalyse the wasteful decomposition of hydrogen peroxidein aqueous solution into oxygen and water.

In view of its decomposition in situ, hydrogen peroxide often representsthe major consumable cost in a pickling or polishing process. Inconsequence, the industry continues to seek ever more effective ways ofreducing the rate and/or extent of the decomposition. In many instances,it has been sought by introducing into solution one or more substancesthat are often called stabilisers, which interact with the metal ionsand/or metal surface and/or the hydrogen peroxide itself in such a wayas to reduce the rate or extent or modify the manner of the interactionsbetween the metal ions and hydrogen peroxide causing decomposition.

There have been many different chemical types of stabilisers proposed oremployed. The literature directed to peroxide stabilisation during metalsurface treatment processes includes many organic compounds asstabilisers such as a range of organic acids or unsaturated aliphaticacids in U.S. Pat. No. 3,537,895 by L. E. Lancy, aromatic alcohols orunsaturated aliphatic alcohols in U.S. Pat. No. 3,869,401 by R. E.Ernst, saturated alcohols in U.S. Pat. No. 3,556,883 by A. Naito et al,amines, amides and imines in U.S. Pat. No. 3,756,957 by S. Shiga, arylsulphonic or sulphamic acids or related compounds in U.S. Pat. No.3,801,512 by J. C. Solenberger et al and solid poorly solublestabilisers like hydroxybenzoic acid in U.S. Pat. No. 4,770,808 by C. F.McDonogh et al. Many other stabilisers have been suggested for peroxidesolutions including substances that chelate the metal ions orprecipitate them out of solution, for example in U.S. Pat. No. 4,059,678to D. C. Winkley. The literature also includes references to inorganicsubstances, such as phosphoric acid in U.S. Pat. No. 3,373,113 toAchenback. Accordingly there is a wide pool of stabilisers from whichthe user can select.

Despite the foregoing, the present inventors found that there remains asignificant problem of stabilising hydrogen peroxide during the metalsurface treatment of steel with aqueous sulphuric acid solutions ofhydrogen peroxide. This is because the greater part of the literaturewas directed to the treatment of copper surfaces and the authorsextrapolated to the treatment of other metals without adequateexperimental support. To some extent, this is demonstrated in U.S. Pat.No. 3,407,141 to R. S. Banush et al, which seeks to etch copper withacidic hydrogen peroxide solutions of long storage life that containcertain urea and aromatic acid compounds. The specification suggeststhat the treatment can be applied to certain other metals but also thatthe solutions are less effective on certain other metals such as . . .stainless steel . . . . Since the patent disclosed results solely withcopper, comments regarding other metals may be regarded simply asspeculation.

In the course of the present research to identify, if possible, asuitable stabiliser system for acidic hydrogen peroxide solutions whichare severely contaminated with dissolved iron, resulting for examplefrom the surface treatment of steels, a large number of comparativestability trials were conducted. Each trial employed a stock solutioncontaining 180 g/l sulphuric acid and 50 g/l hydrogen peroxide and 1%w/w "stabiliser" which was contaminated with 25 g/l dissolved iron fromferric sulphate, and stored at 30° C. or 50° C. Many of the substancestested fell within the classes of stabilisers identified in theabove-mentioned patent specifications.

The trials indicated that many substances which had been described inthe past as stabilisers for hydrogen peroxide in solutions containingonly small amounts of catalytic ions, were unable to prevent rapiddecomposition if substantial iron contamination was present, includingchelating stabilisers like ethylenediaminetetraacetic acid, dipicolinicacid, nitrilotriacetic acid and ethylidene-1-hydroxy-1,1-diphosphonicacid. Moreover, it was found that some substances that acted quite wellas stabilisers when employed separately, acted no better or even lesswell when employed in cogitation under the conditions of the trial.Other combinations of substances demonstrated strictly additivestabiliser properties. Accordingly, the trials demonstrated that adisclosure in a published patent specification that a substance hadstabiliser properties towards hydrogen peroxide under much less extremeconditions or in the presence of copper as the main catalyticcontaminant was no guarantee that it was capable of performingadequately in the presence of a substantial concentration of dissolvediron. The trials also demonstrated that there was no guarantee thatsubstances that had been suggested individually as stabilisers, possiblyguarding against other sources of decomposition, would combine togethereven additively when employed in combination.

SUMMARY OF THE INVENTION

It is an object of the present invention to locate a combination ofsubstances which could stabilise hydrogen peroxide effectively inaqueous sulphuric acid solutions that are employed in the surfacetreatment of steel and therefore become contaminated with significantconcentrations of iron.

According to a first aspect of the present invention there is provided aprocess for stabilising an aqueous solution of hydrogen peroxidecontaining at least 1% w/w sulphuric acid which are suitable fortreating the surface of steel and like alloys characterised in thatthere is introduced into the solution an effective amount, incombination of hydrofluoric acid, hydroxybenzoic acid and anN-alkoxyphenyl-acetamide.

According to a related aspect, there is provided a stabilised aqueoussolution of hydrogen peroxide containing at least 1% w/w sulphuric acidand an effective amount in combination of hydrofluoric acid,hydroxybenzoic acid and an N-alkoxyphenyl-acetamide.

According to a further and related aspect of the present invention thereis provided a process for the surface treatment of steel or a like alloyin which the latter is contacted with an aqueous solution of hydrogenperoxide containing at least 1% w/w sulphuric acid characterised in thatit contains an effective amount in combination of hydrofluoric acid,hydroxybenzoic acid and an N-(alkoxyphenyl)-acetamide.

In the context of the present invention, the stabiliser combinationcomprises hydrofluoric acid, an aromatic acid and an aromatic amide. Thehydroxybenzoic acid is particularly preferably p-hydroxybenzoic acid andthe N-(alkanoxyphenyl)-acetamide, advantageously, contains a lowmolecular weight alkanoxy substituent and especially the compound isN-(4-ethoxy-phenyl)-acetamide.

DESCRIPTION OF PREFERRED EMBODIMENTS

Without being bound to any particular theory, the inventors believe thatthe components of the stabiliser system form a range offluoride-containing complexes with iron and other ions that pass intosolution during surface treatment of steels. The properties of thesecomplexes, and in particular their interaction with hydrogen peroxideare believed to dictate the stability and hence extent of decompositionlosses of hydrogen peroxide during the surface treatments. Furthermore,the presence of hydrofluoric acid is believed to provide the potentialfor the iron complexes to be significantly different from correspondingcomplexes in the absence of hydrofluoric acid and that this may explainto at least some extent why it is so difficult to apply teachings givenfor other solutions in the prior art and teachings on individualcomponents in respect of the combination of the present invention.

The solution preferably contains from 0.5 to 10% w/w hydrofluoric acidand advantageously from 1 to 6%.

The concentrations of the aromatic acid and the aromatic amide insolution are each preferably at least 0.5 g/l and most preferably at ornear saturation. Since they tend to be relatively poorly soluble,saturation can be attained by introduction of about 1 g/l up to a fewg/l of each. The weight ratio of the hydroxy benzoic acid to theacetamide is preferably in the range of 25:1 to 1:5.

In one preferred method of employing the invention stabilisercombination, advantage is taken of the physical properties of the twoaromatic components, namely the acid and amide. In the preferred method,these two components are selected on the bases of their melting pointand solubility. Specifically, it is preferable to select an acid and anamide which does not melt until a temperature significantly in excess ofabout 70° C. is attained and which are scarcely soluble in an aqueousacidic medium. Such compounds will naturally be solids in the normalrange of operating temperatures for hydrogen peroxide-based steelsurface treatments and can dissolve to form a dilute, but saturatedsolution. It is especially desirable to incorporate an excess amount ofthe aromatic acid and amide beyond that needed for a saturated solutionso as to provide within the treatment bath a solid phase, a reservoirwhich can replenish the saturated solution as the compound is removed bythe normal operation of the metal surface treatments, including in situoxidation and by adhesion to the surface of the workpiece on separationfrom the bath. It will be recognised that both the above-named aromaticacid and aromatic amide demonstrate both such preferred characteristics,thereby rendering them especially attractive for this preferred method.

Whilst it is conceivable to incorporate such solids in the metal surfacetreatment solution in powder or granular form, there is a distincttendency for that form of solids to be lost by carry out from a surfacetreatment bath. Powders are not easy to observe, so that it can bedifficult to know how much of the solids are still present and in theextreme case whether any is present at all. Accordingly, in a morepreferred mode of operation, the two solid poorly soluble components ofthe stabiliser system are each employed in the form of a blockcontaining either an individual component or a mixture of them. Theblock is much easier to detect than is the corresponding amount ofpowder or flakes, either visually or by a non-manual system.Accordingly, regulation of the solid stabilisers in the treatment bathcan be accomplished without recourse to elaborate and expensivemonitoring equipment, whilst still minimising the possibility that thebath would be left without the organic components of the stabilisersystem.

The term "block" is used in its normal dictionary meaning, as in U.S.Pat. No. 4,770,808, and covers a wide range of sizes. It typically has aweight of at least 30 g and up to a few kg weight, e.g. 10 kg. For manypractical purposes, it weighs initially from 200 g to 5 kg, but willslowly be consumed during operation of the bath.

The block is normally obtained by compression or binding of flakes,granules or powders into a tablet shape or in some other mould shapesuch as cube, cuboid polyhedron or cylinder, or by resolidification of amelt in such a mould or by extrusion of a rod or bar. Such techniquesare well known in the field of tablet or block formation and accordinglyneed not be described in further detail herein.

The hydrogen peroxide solution is often described as a dilute solution.It normally contains at least 1% w/w hydrogen peroxide and it is unusualfor it to contain more than 10% w/w. For the treatment of steels, it isoften convenient to select within the range of from 3 to 8% w/w hydrogenperoxide. During normal operation, peroxide is consumed, so that withoutcorrective means, its concentration would gradually diminish. At thediscretion of the user, he can seek to maintain a steady state byintroducing peroxide gradually at a rate that matches its consumption,including decomposition, or he can permit the concentration to fluctuateby augmenting the peroxide concentration periodically. The metaltreatment solution is most conveniently obtained by the dilution of aconcentrated commercial hydrogen peroxide solution, typically containingfrom 35 to 70% w/w hydrogen peroxide and trace amounts, i.e. below about0.1% of known storage stabilisers such as pyrophosphate and/or stannateand/or polyphosphonic acid compounds.

The sulphuric acid concentration in the solution is normally not higherthan 20% v/v and in many instances is conveniently selected in theregion of 5 to 15% v/v.

The solution can also include minor amounts of the customary additivesin metal treatment solutions, such as up to about 2% w/w wetting agents.

The processes using the stabilised hydrogen peroxide solutions of thepresent invention are normally carried out at a bath temperature ofabove ambient, and in many instances in the range of from 40° C. to 70°C. Higher temperatures of up to about 80° C. are less often encountered,but become more attractive as a result of the stabilisation of thehydrogen peroxide component in the bath.

The residence period for the work-piece in the treatment bath is at thediscretion of the user and naturally depends on the finish that it isdesired to achieve. Residence periods are often selected in the range offrom 30 seconds to 30 minutes, and normally from 1 to 5 minutes.

The stabilised acidic hydrogen peroxide solutions are primarily intendedfor the pickling or polishing of steels, including mild steel and is ofespecial value for treating stainless steels. Steels suitable fortreatment by the invention process and compositions can contain minorproportions of such metals as chromium, nickle, and manganese; i.e. themetals that are incorporated in corrosion-resistant or stainless steels.

It will be recognised that the process and compositions according to thepresent invention can be employed instead of nitric acid-containingmetal treatment compositions, thereby avoiding the problems of NOxemissions that accompany the use of nitric acid.

Having described the invention in general terms, specific embodimentsthereof will now be described in more detail by way of example only.

EXAMPLE 1 AND COMPARISONS CA TO CC

In this example and these comparisons, the effectiveness of theinvention combination of stabiliser components is compared under thesame conditions of high dissolved iron with stabiliser-free picklingsolution and solution containing components of the combination. In eachtrial, a solution was prepared which contained 5% w/w hydrogen peroxide,10% v/v sulphuric acid, and 1.8% w/w ferric iron, added as ferricsulphate and the stabiliser(s) listed in Table 1 below were then mixedinto the solution. The solid stabilisers are referred to by theirabbreviations; PHBA for p-hydroxybenzoic acid and NEPA forN-(4-ethoxy-phenyl)acetamide. Although the hydrofluoric acid wasentirely miscible with the solution, the solid stabilisers did notdissolve completely, forming a saturated solution of the two compoundsand leaving a residue of solid material.

The solutions were then kept at 30° C. and the residual hydrogenperoxide content was measured at intervals by the standard potassiumpermanganate method. Table 1 below indicates the half life of thehydrogen peroxide in the solution, by which herein we mean the timetaken for the measured hydrogen peroxide to fall to half its initialconcentration.

                  TABLE 1                                                         ______________________________________                                                                    Half-life of                                      Example    Stabiliser System g/l                                                                          H.sub.2 O.sub.2                                   Comp No    HF     PHBA      NEPA  hours                                       ______________________________________                                        CA         --     --        --    2                                           CB         40     --        --    3                                           CC         --     5         5     8                                           Ex1        40     5         5     >300                                        ______________________________________                                    

From Table 1, it can be seen that the effect of employing either the HFalone or the solids alone resulted in some improvement in peroxidestability, but their use in combination resulted in a very substantialimprovement, well mn excess of a simple additive effect. This shows thatthe combination is particularly effective in the presence of substantialconcentrations of iron in solution, as would arise from the surfacetreatment of steel and like alloys.

EXAMPLES 2 TO 5

In these examples, Example 1 was repeated, but using respectively atotal weight of PHBA and NEPA (wt ratio 1:1) of 5 g/l, 10 g/l, 15 g/land 20 g/l, HF at 40 g/l, 12.5% by volume sulphuric acid (98% w/w),about 50 g/l hydrogen peroxide and 29 g/l iron introduced as ferricsulphate. Within the limits of experimental variation, all four amountsof stabilisers resulted in a similar and high proportion of hydrogenperoxide being retained, viz about 82% after 43 hours. This isconsistent with PHBA and NEPA forming saturated solutions at all fourstabiliser amounts tested. To the extent that any trend was apparent,the most efficatious amount was the smallest.

EXAMPLES 6 TO 11

In these examples, a solution was prepared which contained 5% w/whydrogen peroxide, 10% v/v sulphuric acid, 1.8% w/w ferric iron, addedas ferric sulphate and 10 g/l of a mixture of PHBA and NEPA in theproportions by weight listed in Table 2. The solutions were stored at50° C. in order to obtain the comparative results quickly, and residualhydrogen peroxide contents measured at intervals, as for Example 1.Table 2 indicates the percentage remaining after 24 hours.

                  TABLE 2                                                         ______________________________________                                        Ex No   Ratio of NEPA:PHBA                                                                            % peroxide remaining                                  ______________________________________                                        6       2.3:1           46                                                    7       1:1             42                                                    8       1:4             53                                                    9       1:9             43                                                    10       1:24           45                                                    11      9:1             32                                                    ______________________________________                                    

From Table 2, it can be seen that the combination of HF plus NEPA plusPHBA remained a very effective stabiliser over a wide range of ratios ofNEPA to PHBA, and especially in Examples 6 to 10 in which the amount ofeach of the two solid components was sufficient to ensure a saturatedsolution of each.

EXAMPLE 12

In this example, Example 7 was repeated but employing a solutioncontaining additionally chromium at a concentration of 5 g/l, introducedas chromic sulphate. Within the limits of experimental variations, theproportion of hydrogen peroxide remaining in solution after 24 hoursstorage at 50° C. was the same as in the absence of the chromium. Thisdemonstrates that the stabiliser system is applicable for use in thesurface treatment of stainless steels. In other comparative testscarried out with varying additions of chromium to an iron contaminatedsulphuric acid/peroxide solution containing the invention stabilisersystem, there was a tendency for the stabilisation to become somewhatimpaired as the concentration of chromium was increased from 5 to 20g/l.

I claim:
 1. In a method of treating a surface of steel and like alloyswhich comprises contacting said surface with a stabilized aqueous acidichydrogen peroxide solution to alter the appearance of the surface and toimpart thereto a sheen or polish, said solution containing hydrogenperoxide, sulfuric acid, and a stabilizer for reducing decomposition ofthe hydrogen peroxide, the improvement wherein the stabilizer compriseshydrofluoric acid, hydroxybenzoic acid and an N-alkoxyphenyl-acetamide,the amount of each of the hydrofluoric acid, the hydroxybenzoic acid andthe N-alkoxyphenyl-acetamide being such that said stabilizer iseffective to reduce decomposition of the hydrogen peroxide to an extentwhich is greater than the simple additive effect of the hydrofluoricacid, the hydroxybenzoic acid and the N-alkoxyphenyl-acetamide.
 2. Amethod according to claim 1 wherein the hydrogen peroxide concentrationis not more than 10% by weight.
 3. A method according to claim 2 whereinthe hydrogen peroxide concentration is from 1-10% by weight.
 4. A methodaccording to claim 2 wherein the sulfuric acid is present in an amountof from 1 to 20% v/v.
 5. A method according to claim 2 wherein thehydrofluoric acid is present in an amount of 0.5 to 10% w/w.
 6. A methodaccording to claim 5 wherein the hydrofluoric acid concentration is from1-6% w/w.
 7. A method according to claim 2 wherein the hydroxybenzoicacid and the N-alkoxyphenyl-acetamide are each present in an amount ofup to a saturated solution.
 8. A method according to claim 2 whereinsaid hydroxybenzoic acid comprises p-hydroxybenzoic acid.
 9. A methodaccording to claim 8 wherein the N-alkoxyphenyl-acetamide comprisesN-(4-ethoxyphenyl)-acetamide.
 10. In a stabilized aqueous acidichydrogen peroxide composition suitable for use in metal surfacetreatment and containing hydrogen peroxide, sulfuric acid, and astabilizer for reducing decomposition of the hydrogen peroxide, theimprovement wherein the stabilizer comprises hydrofluoric acid,hydroxybenzoic acid and an N-alkoxyphenyl-acetamide, the amount of eachof the hydrofluoric acid, the hydroxybenzoic acid and theN-alkoxyphenyl-acetamide being such that said stabilizer is effective toreduce decomposition of the hydrogen peroxide to an extent which isgreater than the simple additive effect of the hydrofluoric acid, thehydroxybenzoic acid and the N-alkoxyphenyl-acetamide.
 11. A compositionaccording to claim 10 wherein the hydrogen peroxide concentration is notmore than 10% by weight.
 12. A composition according to claim 11 whereinthe hydrogen peroxide concentration is from 1-10% by weight.
 13. Acomposition according to claim 11 wherein the sulfuric acid is presentin an amount of from 1 to 20% v/v.
 14. A composition according to claim11 wherein the hydroxybenzoic acid and the N-alkoxyphenyl-acetamide areeach present in an amount of up to a saturated solution.
 15. Acomposition according to claim 11 wherein the hydroxybenzoic acidcomprises p-hydroxybenzoic acid.
 16. A composition according to claim 11wherein the N-alkoxyphenyl-acetamide comprisesN-(4-ethoxyphenyl)-acetamide.
 17. A composition according to claim 11wherein the weight ratio of the hydroxybenzoic acid to theN-alkoxyphenyl-acetamide is from 5:1 to 25:1.
 18. A compositionaccording to claim 11 wherein the hydroxybenzoic acid and theN-alkoxyphenyl-acetamide are present in the form of a block or blocks.19. A composition according to claim 11 wherein the hydrofluoric acid ispresent in an amount of 0.5 to 10% w/w.
 20. A composition according toclaim 19 wherein the hydrofluoric acid concentration is from 1-6% w/w.21. A stabilizer composition for stabilizing an aqueous acidic hydrogenperoxide solution suitable for metal surface treatment and containinghydrogen peroxide and sulfuric acid, said stabilizer compositioncomprising hydrofluoric acid, hydroxybenzoic acid andN-alkoxyphenyl-acetamide, said stabilizer composition being capable,when added to said acidic hydrogen peroxide solution, of reducing thedecomposition of said hydrogen peroxide to an extent which is greaterthan the simple additive effect of the hydrofluoric acid, thehydroxybenzoic acid, and the N-alkoxyphenyl-acetamide.
 22. A compositionaccording to claim 21 wherein the weight ratio of the hydroxybenzoicacid to the N-alkoxyphenyl-acetamide is from 1:5 to 25:1.
 23. Acomposition according to claim 22 wherein said hydroxybenzoic acidcomprises para-hydroxybenzoic acid.
 24. A composition according to claim23 wherein said N-alkoxyphenyl-acetamide comprisesN-(4-alkoxyphenyl)-acetamide.